In recent years, automobiles have been heightened in power while required to be quiet machines. Moreover, automobiles are used in various environments, and may be used in districts high in external air temperature, such as Southeast Asia or countries in the Middle East or may be used in cold districts. In general, automobiles having an anti-vibration rubber made mainly of a natural rubber and improved in heat resistance encounter no especial problems when used in a district high in external air temperature, but may encounter the following problem when left as they are in a cold district over a long term: a problem that an abnormal vibration is caused when their engine is restarted. This would be caused that when the anti-vibration rubber is exposed to a very low temperature (for example, about −30° C.), the rubber rises in dynamic spring constant to be remarkably lowered in functionality as an anti-vibration rubber. Accordingly, when the quietness of automobiles is considered, it is desired in the actual circumstances to develop a rubber composition for anti-vibration rubber in which a variation in the dynamic spring constant of a rubber is small, in particular, at low temperatures.
As described above, a rubber composition has been generally used which is made mainly of a natural rubber as a rubber component for rubber composition for anti-vibration rubbers. The following technique is known as a technique for decreasing a variation in the dynamic spring constant of a vulcanized rubber made from a rubber composition containing such a rubber component: a technique of increasing the proportion of the amount of a sulfur-based vulcanizing agent in the rubber composition. According to this technique, the anti-vibration rubber tends to be deteriorated in heat resistance so that the rubber does not easily attain compatibility between heat resistance and a restraint of a change with time in the dynamic spring constant in a low-temperature range.
In order to restrain the so-called Mullins effect, i.e., a phenomenon that a rubber is lowered in elastic modulus when repeatedly deformed, Patent Document 1 listed below describes a technique of adjusting, to 50% by mass or more, the proportion by mass of a high-cis isoprene rubber made of a Ziegler catalyst-type isoprene rubber, or a lithium catalyst-type isoprene rubber in all rubber components. However, regarding the invention described in this document, it is assumed that the use of the rubber is for seismic isolation structures, but not assumed in the actual circumstances that the use thereof is in a very low temperature range. Furthermore, a heat resistance level required for the use of the rubber is also lower than that for the use of anti-vibration rubber.